Transmitter for transmitting discovery signals, a receiver and methods therein

ABSTRACT

A transmitter and a method therein for transmitting discovery signals to a receiver. The transmitter and the receiver are comprised in a radio communications system. The transmitter transmits two or more discovery signals over two or more directions. Each discovery signal is configured to span over a fraction of a carrier bandwidth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 14/408,321,now U.S. Pat. No. 9,584,204, which is the national stage filing under 35USC 371 of Application No. PCT/SE2012/050984, filed Sep. 18, 2012, whichclaims the benefit of Application No. 61/668,465, filed Jul. 6, 2012,each of which applications is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

Embodiments herein relate to a transmitter, a receiver and methodstherein. In particular, embodiments herein relate to the transmittal ofdiscovery signals to the receiver.

BACKGROUND

Communication devices such as User Equipments (UE) are enabled tocommunicate wirelessly in a radio communications system, sometimes alsoreferred to as a radio communications network, a mobile communicationsystem, a wireless communications network, a wireless communicationsystem, a cellular radio system or a cellular system. The communicationmay be performed e.g. between two user equipments, between a userequipment and a regular telephone and/or between a user equipment and aserver via a Radio Access Network (RAN) and possibly one or more corenetworks, comprised within the wireless communications network.

User equipment are also known as e.g. mobile terminals, wirelessterminals and/or mobile stations, mobile telephones, cellulartelephones, or laptops with wireless capability, just to mention someexamples. The user equipments in the present context may be, forexample, portable, pocket-storable, hand-held, computer-comprised, orvehicle-mounted mobile devices, enabled to communicate voice and/ordata, via the RAN, with another entity.

The wireless communications network covers a geographical area which isdivided into cell areas, wherein each cell area being served by anetwork node such as a Base Station (BS), e.g. a Radio Base Station(RBS), which sometimes may be referred to as e.g. eNB, eNodeB, NodeB, Bnode, or BTS (Base Transceiver Station), depending on the technology andterminology used. The base stations may be of different classes such ase.g. macro eNodeB, home eNodeB or pico base station, based ontransmission power and thereby also cell size. A cell is thegeographical area where radio coverage is provided by the base stationat a base station site. One base station, situated on the base stationsite, may serve one or several cells. Further, each base station maysupport one or several radio access and communication technologies. Thebase stations communicate over the radio interface operating on radiofrequencies with the user equipments within range of the base stations.

In some RANs, several base stations may be connected, e.g. by landlinesor microwave, to a radio network controller, e.g. a Radio NetworkController (RNC) in Universal Mobile Telecommunications System (UMTS),and/or to each other. The radio network controller, also sometimestermed a Base Station Controller (BSC) e.g. in GSM, may supervise andcoordinate various activities of the plural base stations connectedthereto. GSM is an abbreviation for Global System for MobileCommunications (originally: GroupeSpécial Mobile).

In the context of this disclosure, the expression Downlink (DL) is usedfor the transmission path from the base station to the user equipment.The expression Uplink (UL) is used for the transmission path in theopposite direction i.e. from the user equipment to the base station.

In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE),base stations, which may be referred to as eNodeBs or even eNBs, may bedirectly connected to one or more core networks.

UMTS is a third generation mobile communication system, which evolvedfrom the GSM, and is intended to provide improved mobile communicationservices based on Wideband Code Division Multiple Access (WCDMA) accesstechnology. UMTS Terrestrial Radio Access Network (UTRAN) is essentiallya radio access network using wideband code division multiple access foruser equipments. The 3GPP has undertaken to evolve further the UTRAN andGSM based radio access network technologies.

According to 3GPP/GERAN, a user equipment has a multi-slot class, whichdetermines the maximum transfer rate in the uplink and downlinkdirection. GERAN is an abbreviation for GSM EDGE Radio Access Network.EDGE is further an abbreviation for Enhanced Data rates for GSMEvolution.

The past 30 years have seen a tremendous improvement in the state ofInformation and Communication Technologies (ICT), formally led by theComputing and the Telecommunications industries. This improvement ismost felt in the increase in global Internet traffic, which has beenconservatively predicted to reach a ten-fold growth from 2010 levels by2016. Other forecasts by Cisco predict an increase in traffic of as muchas a 92% cumulative annual growth rate; this amounts to a 700-foldincrease in traffic by 2020.

A majority of this traffic growth is expected to come from the increasedconsumption of video on mobile networks, as well as a net increase insubscribers transitioning to mobile broadband even as the fixed andmobile networks converge to provide end-user experience that isindistinguishable in many environments. Added to this, it has beenpredicted that the mobile broadband industry will get most of its growthin the number of connections from the widespread introduction of MachineType Communication (MTC) devices that will drive the Machine-to-Machine(M2M) market for applications from diverse industries such as Utilities(e.g. Smart Grid), Automotive (e.g. Intelligent Transportation), Healthcare. Apart from these industries, the broad area of IndustrialAutomation is expected to create new business opportunities in a varietyof industries such as Agriculture, Mining and Exploration, Oil andNatural Gas Distribution, Residential and Building Automation etc.Estimates of the number of devices vary widely from our own declamationof an increase from 5 billion subscriptions to 50 billion connecteddevices.

One key development that is inevitable is a merging of fixed andwireless networks in what has been termed as the Fixed MobileConvergence (FMC).

There is still some scope for a part of the predicted traffic increaseto happen due to network build out in areas of the world not covered bymobile broadband. However, it is also true that much of the increase indata traffic will happen based on the kind of activities people engagein over the Internet, such as the transition of video services frombroadcast networks to online video sources. This leads to our convictionthat the bulk of Internet traffic increase will happen in areas that arealready served by cellular networks.

Table 1 below is a generational classification of broadband cellulartechnologies. The table uses an accepted and correct technicalclassification, while it is acknowledged that industry and media mayoften use a more sensational approach to distinguishing a generation.With the introduction of LTE and all indications of LTE being the solesurviving cellular standard, it is now possible to identify a trueconvergence of mobile radio technologies.

TABLE 1 generational classification of broadband cellular technologies.The data rates are in orders of magnitude and the numbers areapproximations. International Telecommunication International MobileUnion (ITU) Telecommunications 2000 IMT- Classification/ (IMT-2000)Advanced Generation 1G 2G 3G 4G Technology AMPS/NMT GSM/EDGE WCDMA/HSPA3GPP LTE Examples (as CDMA2000/evDO IEEE component of WiMAX rel. 1.1802.16-2009 EIA/TIA-136) EIA/TIA-95 Type Analog Digital Digital DigitalChannelization <100 KHz <1 MHz <10 MHz <100 MHz Frequency band 400-1000MHz 400-2000 MHz 400-3000 MHz 200-5000 MHz Data rates <10 kb/s/user <1Mb/s/cell <100 Mb/s/cell <1 Gb/s/cell Services Voice Voice/dataVoice/Data Data telephony (voice included)

The US National Broadband Plan aims to create new allocations formobile, fixed and unlicensed broadband access of up to 500 MHz ofspectrum below 5 GHz by 2020 (FCC, “Connecting America: The NationalBroadband Plan,” at http://www.broadband.gov, March 2010.) Currently 547MHz has been designated as flexible use spectrum for wireless broadband,of which roughly 170 MHz is available to cellular and PersonalCommunications Service (PCS) operators. With the existing allocations of547 MHz of spectrum including the recent Advanced Wireless Services-1(AWS-1) auctions, this should give the mobile industry over 800 MHz ofspectrum to improve their ability to handle more users and newerservices. Even with such largesse, it is inconceivable that systemcapacity for cellular networks will improve by an order of magnitude inthe future without significant reengineering of the way networks aredeployed.

It should be noted that the lack of spectrum has driven wireless networkdeployment in two directions.

Firstly, every system has improved throughputs as well as spectralefficiency over the previous generation using a variety of technologicalapproaches such as

-   -   a reduction in cell size through densification of the network,        the development of Heterogeneous networks (hetnets) as a means        of boosting capacity and bitrates,    -   deployment of additional spectrum,    -   packet data based on the Internet Protocol (IP),    -   wider bandwidths,    -   link adaptation using adaptive modulation and coding, and        Hybrid-Automatic Repeat reQuest (HARQ),    -   higher order modulation schemes,    -   antenna techniques such as beamforming and Multi-Input        Multi-Output (MIMO),    -   advanced receiver architectures such as Successive Interference        Cancellation (SIC), multi-stage SIC, joint demodulation,    -   advanced network procedures such as interference coordination.

These techniques have provided the means to increase peak spectralefficiencies per link to as much as 15 b/s/Hz. Of course, the observedcell spectral efficiencies vary according to the radio environment andthe interference level and typically are of the order of 1-3 b/s/Hz onaverage.

Secondly, systems such as LTE that may operate over channel bandwidthsof up to 100 MHz do so with the aid of carrier aggregation. Aggregationof carriers cannot be done arbitrarily and radio requirements becomevery complicated when specifying the particular combinations of carrierbandwidths that may be used to populate a band or combined across bands.

Given the state of spectrum allocations for mobile systems, it is ofinterest to see if the evolution of modern mobile networks may proceedbeyond 4G. The objective of such an evolution would be to improve datarates by yet another order of magnitude over the last generation, and tomoreover do this under the assumption of a dense deployment ofinfrastructure nodes providing radio links to mobile users. Such anetwork would also need to do be deployed with much larger spectrumallocations, typically operating under conditions of low to moderatemobility. The reach of such a network would span indoor locations aswell as densely populated urban centers.

Today's cellular communication occurs largely in frequency bands below 3GHz in what we term as an interference-limited environment. While LTEmay operate over bandwidths of as much as 100 MHz by design, the futureradio access system we envisage would operate over bandwidths of theorder of 1 GHz. Clearly, such a system could not operate in bands below3 GHz. The lowest band where the mobile industry may home for spectrumparcels that exceed the 10-40 MHz of contiguous allocations typical forthe industry is probably above 3 GHz. Of the regions of spectrum thatare most promising for the mobile industry, the cm-Wave (CMW) regionfrom 3-30 GHz and the mm-Wave (MMW) region from 30-300 GHz may beconsidered as being particularly interesting for the next generationmobile systems.

Table 2 is a link budget for a pair of radios that are configured tooperate in two modes. By the term “radios” when used herein is meantdevices comprising both transmission and reception functions. The firstmode is a low data rate mode using low antenna gain and the second modeis a high data rate mode using high antenna gain. It is well known thatsuch a variation in antenna gain may be obtained by using active antennasolutions composed of many antennas integrated with a set of radiochains that are at most equal in number to the number of antennaelements. The conducted power from the transmitters are transferred tothe antenna elements through a transfer matrix that may adjust the phaseand optionally the amplitude of the transmitter outputs so as to createa resultant directivity pattern for the antenna array that may eitherhave a high gain or a low one. Typically, the tradeoff in such anarrangement is between the spatial region covered by the antenna, whichis large for a low effective array gain, and is narrow when a high gainis chosen.

TABLE 2 Link budget at 40 GHz for a pair of radios that is able to tradeoff data rate for antenna gain. Case 1 Case 2 Parameter link budget Linkbudget Tx Power: P_TX 20 dBm 20 dBm Tx Antenna Gain: G_T 2 dB 14 dBEquivalent Isotropically 22 dBm 34 dBm Radiated Power (EIRP) Bandwidth 1GHz 1 GHz Noise power: kTB −84 dBm −84 dBm Rx Antenna Gain: G_R 0 dB 14dB Receiver Noise Figure (NF) 7 dB 7 dB Receiver Sensitivity Margin 2 dB2 dB Shadowing margin 3 dB 3 dB Carrier-to-Noise Ratio (C/N) −3 dB 14 dBneeded (0.1 b/s/Hz) (5 b/s/Hz) Required received power −72 dBm −61 dBmPath loss budget 94 dBm 95 dBm Range (40 GHz) 30 m 30 m A free spacepropagation loss model has been used for illustrative purposes.Shadowing losses are meant to represent all possible additionalpropagation losses. Some common losses such as those at the transmit andreceive switch may have been ignored.

In a system that transmitter wise relies on narrow beams to obtain therequired link budget a problem is to enable a receiving device to findthe transmitting system, i.e. to make the receiving device aware of thepresence of the system.

In traditional cellular systems such a signal is typically transmittedwith a very wide beam pattern thus enabling receiving devices in thecoverage area of the wide beam to detect the system.

In a system that relies on high-gain transmitter beamforming to achievethe required link budget a wide beam may not convey sufficient energyinto a given direction for a receiving device to detect the system. InIEEE 802.11ad standard this problem is solved by beamforming a widebanddiscovery signal into one particular direction and in a Time DivisionMultiplexing (TDM) fashion cycle through different transmit directionsto cover the complete area of interest.

A problem is that this solution cannot be applied to an envisionedSuper-Densed Network (SDN) wherein also narrowband devices should becapable of accessing the system since they cannot receive a widebanddiscovery signal.

SUMMARY

An object of embodiments herein is to provide a way of improving theperformance in a communications network.

According to a first aspect of embodiments herein, the object isachieved by a method in a transmitter for transmitting discovery signalsto a receiver. The transmitter and the receiver are comprised in a radiocommunications system. The transmitter transmits two or more discoverysignals over two or more directions, wherein the each discovery signalis configured to span over a fraction of a carrier bandwidth.

According to a second aspect of embodiments herein, the object isachieved by a transmitter for transmitting discovery signals to areceiver. The transmitter and the receiver are comprised in a radiocommunications system. The transmitter is configured to transmit two ormore discovery signals over two or more directions, wherein eachdiscovery signal is configured to span over a fraction of a carrierbandwidth.

According to a third aspect of embodiments herein, the object isachieved by a method in a receiver for receiving discovery signals froma transmitter. The transmitter and the receiver are comprised in a radiocommunications system. The receiver receives from the transmitter atleast one of two or more discovery signals that have been transmittedinto two or more directions wherein each discovery signal is configuredto span over a fraction of a carrier bandwidth.

According to a fourth aspect of embodiments herein, the object isachieved by a receiver for receiving discovery signals from atransmitter. The transmitter and the receiver are comprised in a radiocommunications system. The receiver is configured to receive from thetransmitter at least one of two or more discovery signals that have beentransmitted into two or more directions, wherein each discovery signalis configured to span over a fraction of a carrier bandwidth.

Since two or more discovery signals are transmitted over two or moredirections and since each discovery signal spans over only a fraction ofthe carrier bandwidth, the receiver is enabled to detect a widebandcarrier even if the receiver is a narrowband receiver, i.e. a receiversupporting only a fraction of the total carrier bandwidth. This resultsin an improved performance in the communications network.

An advantage of embodiments herein is that a narrowband receiver isenabled to camp on a sub-band without first camping in the center of awideband carrier, which center may be unknown to the narrowband receiveron beforehand.

A further advantage of embodiments herein is that a wideband receiver isenabled to scan multiple directions at once thus speeding up thediscovery process.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments herein are described in more detail withreference to attached drawings in which:

FIG. 1 is a schematic block diagram illustrating embodiments of acommunications system;

FIG. 2a is a flowchart depicting embodiments of a method in atransmitter;

FIG. 2b is a schematic block diagram illustrating embodiments of atransmitter;

FIG. 3a is a flowchart depicting embodiments of a method in a receiver;

FIG. 3b is a schematic block diagram illustrating embodiments of areceiver;

FIGS. 4a and 4b schematically illustrate active antenna arrays that arebeamformed into low gain and large coverage, and high gain and narrowcoverage, respectively;

FIGS. 5a, 5b, and 5c schematically illustrate embodiments of transmitter(FIGS. 5a,5b ) and receiver (FIG. 5c ) representations for a fourantenna element linear array with two antenna ports;

FIG. 6 is a schematic representation of a 16 element planar antennaarray;

FIG. 7 schematically illustrates discovery signals for differentdirections (sectors) multiplexed in an FDM fashion;

FIG. 8 schematically illustrates discovery signals for differentdirections multiplexed in frequency;

FIG. 9 schematically illustrates discovery signals for differentdirections multiplexed in frequency and repeated over time;

FIGS. 10a and 10b schematically illustrate discovery signals fordifferent directions multiplexed in frequency and repeated over time. InFIG. 10a , the discovery signals are repeated twice immediately aftereach other, and in FIG. 10b , the discovery signals are rotated andtransmitted periodically;

FIG. 11 schematically illustrates discovery signals for differentdirections multiplexed in TDM fashion and transmitted over the samesub-band; and

FIGS. 12a and 12b schematically illustrate discovery signals fordifferent directions multiplexed in TDM fashion and transmitted overdifferent sub-bands.

DETAILED DESCRIPTION

Embodiments herein will be exemplified in the following non-limitingdescription.

As part of developing embodiments herein, a problem will first beidentified and discussed.

It is proposed to create the next standard to operate over bandwidthsthat range from 100 MHz to 2.5 GHz in dense deployment and overfrequency bands that allow the use of beamforming to establish nearLine-of-Sight (LoS) links between communicating radios.

The resulting system may be used in a variety of scenarios, e.g.Point-to-point communications for short range radio systems such asAccess to Network links for a Future Radio Access (FRA) system thatprovides very high speed wide area connectivity, and backhaul linksbetween densely deployed infrastructure nodes that provide a highthroughput pipeline to a network operator's core network, which corenetwork may connect to the Internet and provide access to data andmultimedia services.

Competing standards for Millimeter Wave (MMW) communications, e.g. theIEEE 802.11ad standard, operate over the whole channel bandwidth. In theexample of the IEEE 802.11ad standard, the whole channel bandwidth is 2GHz. A design for a Super-Dense Network (SDN) operating in theMillimeter Wave bands should however enable devices supporting less thanthe full channel bandwidth to find and operate on a wider carrier, e.g.a device supporting 200 MHz should be able to operate on a 2 GHz widecarrier. The IEEE 802.11ad standard and other MMW standards did not havethis design guideline and therefore the developed discovery signals arenot applicable for the SDN system.

FIG. 1 schematically illustrates embodiments of a radio communicationssystem 100. The radio communication system 100 may be a 3GPPcommunications system or a non-3GPP communications system. The radiocommunications system 100 may comprises one or more of radiocommunications networks (not shown). Each radio communications networkmay be configured to support one or more Radio Access Technologies(RATs). Further, the one or more radio communications networks may beconfigured to support different RATs. Some examples of RATs are GSM,WCDMA, and LTE.

The radio communications system 100 comprises a radio network node 102.The radio network node 102 may be a base station such as an eNB, aneNodeB, Node B or a Home Node B, a Home eNode B, a radio networkcontroller, a base station controller, an access point, a relay nodewhich may be fixed or movable, a donor node serving a relay, a GSM/EDGEradio base station, a Multi-Standard Radio (MSR) base station or anyother network unit capable to serve a user equipment or another radionetwork node comprised in the cellular communications system 100.

Further, it should be understood that the radio network node 102 is oneexample of an access node (not shown) comprised in the radiocommunication system 100. When used herein the term “access node”represents a transition between the access link (between the userequipment and a network owned resource) and the backhaul which istypically confined to operator or network owned and managed resources.Embodiments herein may be applied to sets of peer radio entities as welland is equally applicable to an ad hoc network composed of user entitiesalone, where a master radio may send a discovery signal and a slavedevice may try detecting the discovery signal, without specifying therole of a master or slave to any particular device type.

The radio communications system 100 comprises further a transmitter 103.The transmitter 103 may be comprised in the radio network node 102.However, it should be understood that the transmitter 103 may becomprised in any other access node.

Further, the radio network node 102 provides radio coverage over atleast one geographical area 104, which herein sometime is referred to asa cell 104.

The radio communications system 100 comprises further a user equipment106. The user equipment 106 is located within the cell 104 and is servedby the radio network node 102. Further, the user equipment 106 transmitsdata over a radio interface to the radio network node 102 in an uplink(UL) transmission and the radio network node 102 transmits data to theuser equipment 106 in a downlink (DL) transmission.

The first user equipment 106 may be e.g. a mobile terminal or a wirelessterminal, a mobile phone, a computer such as e.g. a laptop, a tablet pcsuch as e.g. an iPad™, a Personal Digital Assistant (PDA), or any otherradio network unit capable to communicate over a radio link in acellular communications network. The first user equipment 106 mayfurther be configured for use in both a 3GPP network and in a non-3GPPnetwork.

The radio communications system 100 comprises further a receiver 107.The receiver 107 may be comprised in the user equipment 106 or in a nodesuch as an access node.

Further, it should be understood that the access node, such as radionetwork node, and the user equipment may each comprise both atransmitter and a receiver. Thus, in some embodiments, the receiver 107may be comprised in a second access node, such as a second radio networknode (not shown).

Embodiments herein relates to different designs of a directionaldiscovery signal.

A first design multiplexes in the frequency-domain discovery signals formultiple directions into the same symbol. Each discovery signal spansonly a fraction of the total carrier bandwidth and thus enabling narrowbandwidth devices to detect the wideband carrier.

FIG. 7 schematically illustrates how discovery signals for differentdirections, i.e. sectors, are multiplexed in a Frequency-DivisionMultiplexing (FDM) fashion. Each discovery signal spans over only afraction of the total carrier bandwidth to enable narrow-band devicescomprising a receiver 107 to find the discovery signal. By means of thereceived discovery signal, the receiver 107 detects the transmitter 103.Further, by means of the received discovery signal, the receiver 107 maydetect the radio communications system 100. A part of the total carrierbandwidth may be divided into a number of sub-bands. FIG. 7 will befurther discussed below.

In a second design only one discovery signal is transmitted in a symbol.As in the first design this enables narrow bandwidth devices to detectthe wideband carrier. Compared to the first design, the second designprovides for an increase in the power radiated into a given directionand thereby increases the coverage area.

A method in the transmitter 103 for transmitting discovery signals to areceiver 107 will now be described with reference to FIG. 2 a.

As previously mentioned, the transmitter 103 and the receiver 107 arecomprised in the radio communications system 100.

The method comprises the following actions, which do not have to beperformed in the order stated below, but may be taken in any suitableorder. Further, actions may be combined.

Action 201

In order for the receiver 107 to discover the transmitter 103, thetransmitter 103 transmits at least one discovery signal, e.g. two ormore discovery signals, over one or more directions, e.g. two or moredirections. The one or more directions may be selected from apredetermined set of possible directions. The at least one discoverysignal may be comprised in a symbol. In some embodiments, two or morediscovery signals are transmitted in a single symbol. However, two ormore discovery signals may be transmitted in two or more symbols. Thesymbol may be an OFDM symbol, but it may also be a symbol according toother OFDM-like schemes such as pulse-shaped OFDM, Isotropic OrthogonalTransform Algorithm OFDM (IOTA-OFDM), filter-bank OFDM, waveletmodulation, etc, or any system that separates its carrier bandwidth intomultiple sub-bands. Further, the at least one discovery signal isconfigured to span over a fraction of a carrier bandwidth. Thereby, thereceiver 107 may detect a wide bandwidth carrier even if the receiver107 is a narrow bandwidth receiver.

The at least one discovery signal may be a synchronisation signal.

The carrier bandwidth may be separated into sub-bands. In someembodiments, a part of the carrier bandwidth is separated intosub-bands. Further, the carrier bandwidth may be in the range from 100MHz to 2.5 GHz. Such a carrier bandwidth is herein sometimes referred toas a wide carrier bandwidth. Furthermore, the at least one discoverysignal may be configured to span over one sub-band.

In some embodiments, the transmitter 103 multiplexes in thefrequency-domain a plurality of discovery signals for a plurality ofdirections, and transmits the multiplexed plurality of discovery signalsin the single symbol or in two or more symbols.

The transmitter 103 may transmit the at least one discovery signaltogether with information relating to at least one of: informationindicating a beam direction, i.e. a signal direction, informationindicating a frequency offset to the carrier centre and informationindicating the transmitter 103.

Further, the transmitter 103 may transmit the at least one discoverysignal into an independent direction. By the expression “independentdirection” when used herein is meant a direction that is different froma direction of a second discovery signal. The second discovery signalmay be the same discovery signal as a first discovery signal or it maybe a discovery signal different from the first discovery signal.

The transmitter 103 may further, over time, cyclically transmit the atleast one discovery signal into different directions.

In some embodiments, the location of the signaling sub-band may not beknown to the narrowband devices, e.g. the receivers 107, beforehand, andthe radio front-ends of some narrowband devices, e.g. receivers 107, mayonly be able to receive from a limited number of sub-bands. Thereforethe transmitter 103 may periodically change the frequency location ofthe signaling sub-band, whereby each narrowband device may be allowed todetect the discovery signal by listening to a fixed sub-band all thetime.

To perform the method action in the transmitter 103 described above inrelation to FIG. 2a , the transmitter 103 may comprise the followingarrangement depicted in FIG. 2 b.

As previously mentioned, the transmitter 103 and the receiver 107 arecomprised in a radio communications system 100.

The transmitter 103 comprises an input and output interface 210configured to function as an interface for communication in thecommunication system 100. The communication may for example becommunication with the receiver 107.

The transmitter 103 is configured to transmit at least one discoverysignal, e.g. two or more discovery signals, over one or more directions,e.g. two or more directions. The one or more directions may be selectedfrom a predetermined set of possible directions. The at least onediscovery signal may be comprised in a symbol. In some embodiments, twoor more discovery signals are transmitted in a single symbol. However,two or more discovery signals may be transmitted in two or more symbols.As previously mentioned, the symbol may be an OFDM symbol, but it mayalso be a symbol according to other OFDM-like schemes such aspulse-shaped OFDM, IOTA-OFDM, filter-bank OFDM, wavelet modulation, etc,or any system that separates its carrier bandwidth into multiplesub-bands. The transmitter 103 may comprise a transmitting circuit 212configured to transmit the at least one discovery signal. Further, theat least one discovery signal is configured to span over a fraction of acarrier bandwidth. Thereby, the receiver 107 may detect a wide bandwidthcarrier even if the receiver 107 is a narrow bandwidth receiver.

The at least one discovery signal may be a synchronisation signal.

The carrier bandwidth may be separated into sub-bands. In someembodiments, a part of the carrier bandwidth is separated intosub-bands. Further, the carrier bandwidth may be in the range from 100MHz to 2.5 GHz. Furthermore, the at least one discovery signal may beconfigured to span over one sub-band.

In some embodiments, the transmitter 103 is further configured tomultiplex in the frequency-domain a plurality of discovery signals for aplurality of directions, and to transmit the multiplexed plurality ofdiscovery signals in the single symbol or in two or more symbols. Thetransmitter 103 may comprise a multiplexing circuit (not shown)configured to perform the multiplexing.

The transmitter 103 may further be configured to transmit the at leastone discovery signal together with information relating to at least oneof: information indicating a beam direction, information indicating afrequency offset to the carrier centre and information indicating thetransmitter 103. In some embodiments, the transmitting circuit 212 isconfigured to transmit the at least one discovery signal together withinformation relating to at least one of: information indicating a beamdirection, information indicating a frequency offset to the carriercentre and information indicating the transmitter 103.

Further, the transmitter 103 may be configured to transmit the at leastone discovery signal into an independent direction. As previouslymentioned, by the expression “independent direction” when used herein ismeant a direction that is different from a direction of a seconddiscovery signal. The second discovery signal may be the same discoverysignal as a first discovery signal or it may be a discovery signaldifferent from the first discovery signal.

In some embodiments, the transmitter 103 is configured to, over time,cyclically transmit the at least one discovery signal into differentdirections.

Embodiments herein for transmitting discovery signals to a receiver 107may be implemented through one or more processors, such as a processingcircuit 214 in the transmitter 103 depicted in FIG. 2b , together withcomputer program code for performing the functions and/or method actionsof embodiments herein.

It should be understood that one or more of the circuits comprised inthe transmitter 103 described above may be integrated with each other toform an integrated circuit.

The transmitter 103 may further comprise a memory 216. The memory maycomprise one or more memory units and may be used to store for exampledata such as thresholds, predefined or pre-set information, etc.

A method in a receiver 107 for receiving discovery signals from atransmitter 103 will now be described with reference to FIG. 3 a.

As previously mentioned, the transmitter 103 and the receiver 107 arecomprised in the radio communications system 100.

The method comprises the following actions, which do not have to beperformed in the order stated below, but may be taken in any suitableorder. Further, actions may be combined.

Action 301

The receiver 107 receives, from the transmitter 103, at least onediscovery signal, e.g. two or more discovery signals, from one or moredirections, e.g. two or more directions. The at least one discoverysignal received by the receiver 107 may be at least one of two or morediscovery signals transmitted from the transmitter 103. The at least onediscovery signal may be comprised in a symbol. In some embodiments, twoor more discovery signals are received in a single symbol. However, twoor more discovery signals may be received in two or more symbols. Aspreviously mentioned, the symbol may be an OFDM symbol, but it may alsobe a symbol according to other OFDM-like schemes such as pulse-shapedOFDM, IOTA-OFDM, filter-bank OFDM, wavelet modulation, etc, or anysystem that separates its carrier bandwidth into multiple sub-bands.Further, the at least one discovery signal is configured to span over afraction of a carrier bandwidth. Thereby, the receiver 107 may detect awide bandwidth carrier even if the receiver 107 is a narrow bandwidthreceiver

The at least one discovery signal may be a synchronisation signal.

The carrier bandwidth may be separated into sub-bands. In someembodiments, a part of the carrier bandwidth is separated intosub-bands. Further, the carrier bandwidth may be in the range from 100MHz to 2.5 GHz. Furthermore, the at least one discovery signal may beconfigured to span over one sub-band.

In some embodiments, the receiver 107 receives the at least onediscovery signal together with information relating to at least one of:information indicating a beam direction, information indicating afrequency offset to the carrier centre and information indicating thetransmitter 103.

The receiver 107 may further receive the at least one discovery signalfrom an independent direction.

In some embodiments, the receiver 107 cyclically, over time, receivesthe at least one discovery signal that has been transmitted intodifferent directions.

In some embodiments, the location of the signaling sub-band may not beknown to the narrowband device, e.g. the receiver 107, beforehand, andthe radio front-ends of some narrowband devices, e.g. receivers 107, mayonly be able to receive from a limited number of sub-bands. Thereforethe transmitter 103 may periodically change the frequency location ofthe signaling sub-band, whereby the receiver 107 may be allowed todetect the discovery signal by listening to a fixed sub-band all thetime.

To perform the method action in the receiver 107 described above inrelation to FIG. 3a , the receiver 107 comprises the followingarrangement depicted in FIG. 3 b.

As previously mentioned, the transmitter 103 and the receiver 107 arecomprised in the radio communications system 100.

The receiver 107 comprises an input and output interface 310 configuredto function as an interface for communication in the communicationsystem 100. The communication may for example be communication with thetransmitter 103.

The receiver 107 is configured to receive, from the transmitter 103, atleast one discovery signal that has been transmitted into one or moredirections. Further, the receiver 107 may be configured to receive, fromthe transmitter 103, at least one of two or more discovery signals thathave been transmitted into two or more directions. The at least onediscovery signal may be comprised in a symbol. In some embodiments, twoor more discovery signals are received in a single symbol. However, twoor more discovery signals may be received in two or more symbols. Aspreviously mentioned, the symbol may be an OFDM symbol, but it may alsobe a symbol according to other OFDM-like schemes such as pulse-shapedOFDM, IOTA-OFDM, filter-bank OFDM, wavelet modulation, etc, or anysystem that separates its carrier bandwidth into multiple sub-bands. Thereceiver 107 may comprise a receiving circuit 312 configured to receivethe at least one discovery signal. Further, the at least one discoverysignal is configured to span over a fraction of a carrier bandwidth.

The at least one discovery signal may be a synchronisation signal.

The carrier bandwidth may be separated into sub-bands. In someembodiments, a part of the carrier bandwidth is separated intosub-bands. Further, the carrier bandwidth may be in the range from 100MHz to 2.5 GHz. Furthermore, the at least one discovery signal may beconfigured to span over one sub-band.

In some embodiments, the receiver 107 is further configured to receivethe at least one discovery signal together with information relating toat least one of: information indicating a beam direction, informationindicating a frequency offset to the carrier centre and informationindicating the transmitter 103. The receiving circuit 312 may beconfigured to receive the at least one discovery signal together withinformation relating to at least one of: information indicating a beamdirection, information indicating a frequency offset to the carriercentre and information indicating the transmitter 103.

The receiver 107 may be configured to receive the at least one discoverysignal from an independent direction.

Further, the receiver 107 may be configured to, over time, cyclicallyreceive the at least one discovery signal that has been transmitted intodifferent directions. Embodiments herein for receiving a discoverysignal from a transmitter 103 may be implemented through one or moreprocessors, such as a processing circuit 314 in the receiver 107depicted in FIG. 3b , together with computer program code for performingthe functions and/or method actions of embodiments herein.

It should be understood that one or more of the circuits comprised inthe receiver 107 described above may be integrated with each other toform an integrated circuit.

The receiver 107 may further comprise a memory 316. The memory maycomprise one or more memory units and may be used to store for exampledata such as thresholds, predefined or pre-set information, etc.

FIGS. 4a and 4b schematically illustrate active antenna arrays that arebeamformed into low gain and large coverage, and high gain and narrowcoverage, respectively. The active antenna arrays may be comprised inembodiments described herein.

FIGS. 5a, 5b, and 5c schematically illustrate embodiments of thetransmitter 103 (FIGS. 5a,5b ) and the receiver 107 (FIG. 5c )comprising a four antenna element linear array with two antenna ports.Adjusting the phase and gain of the antenna elements may shape and steerthe resulting antenna pattern. The antenna elements are spaced afraction of a wavelength apart. In FIGS. 5a-5c , some components such asfilters have been omitted.

FIG. 5a schematically illustrates embodiments of the transmitter 103when having an analog gain and phase adjustment. The transmitter 103comprises four antenna elements 501 a. Further, the transmitter 103comprises four phase adjustment circuits 503 to which the antennaelements 501 a are connected. In FIG. 5a , a ground plane 502 a isillustrated, which ground plane 502 a is configured to function as amirror to avoid beams back to the transmitter 103. The transmitter 103comprises also two Power Amplifiers (PA) 504 a to each of which a pairof phase adjustment circuits 503 is connected. A signal from thetransmit chain will be power amplified at the PA 504 a and thenseparated into two signals and fed to respective phase adjustmentcircuit 503. The respective phase adjustment circuit 503 will adjust thephase of the respective signals. Thereafter the two signals will betransmitted from the respective antenna elements 501 a.

FIG. 5b schematically illustrates embodiments of the transmitter 103having digital gain and phase adjustment. The transmitter 103 comprisesfour antenna elements 501 b. Further, the transmitter 103 comprises twoPower Amplifiers (PA) 504 b to each of which a pair of antenna elements501 b is connected. In FIG. 5b , a ground plane 502 b is illustrated,which ground plane 502 b is configured to function as a mirror to avoidbeams back to the transmitter 103. The ground plane 502 b is comprisedin the transmitter 103. Each PA 504 b is connected to a mixer 505 bcomprised in the transmitter 103. Further, the transmitter 103 comprisestwo Digital-to-Analog converters (D/A) 506 and two gain and phaseadjustment circuits 507 b. Each gain and phase adjustment circuit 507 bis connected to a respective mixer 505 b via a respective D/A 506.

FIG. 5c schematically illustrates embodiments of the receiver 107 withreceive beamforming. It should be understood that when the gain andphase of signals collected by a set of antennas are adjusted and thencombined after the gain and phase adjustment, the resultant combinedsignal may appear as if from a particular set of spatial directions. Ifthe gain and phase adjustment was made in a way where the receiverassumption of direction matches with the actual received signal, receivebeamforming is obtained.

As illustrated in FIG. 5c , the receiver 107 comprises four antennaelements 501 c, a ground plane 502 c and four Low Noise Amplifiers (LNA)508. Each antenna element 501 c is connected to a respective LNA 508.The ground plane 502 c is configured to function as a mirror to avoidbeams back to the receiver 107. The receiver 107 comprises further fourmixers 505 c, four Analog-to-Digital converters (A/D) 509 and four gainand phase adjustment circuits 507 c. Each gain and phase adjustmentcircuit 507 c is connected to a respective mixer 505 c via a respectiveD/A 509. Further, the transmitter comprises a combiner 510 to which eachof the gain and phase adjustment circuits 507 c is connected. Thesignals received from the antenna elements 501 c are first amplified byLNA 508, downconverted into baseband signals by the mixer 505 c,digitized by A/D 509, and then scaled accordingly by gain and phaseadjustment circuits 507 c before being (coherently) combined into asingle baseband signal.

Further, FIG. 6 is a schematic representation of a 16 element planarantenna array that may be comprised in embodiments described herein,such as the receiver and the transmitter. As mentioned above, theantenna elements 601 are spaced a fraction of a wavelength apart. InFIG. 6, the antenna elements 601 are spaced half a wavelength a part,i.e. lambda(λ)/2. However, it should be understood that they don't haveto be spaced half a wavelength a part, i.e. lambda(λ)/2.

Some embodiments will now be described in more detail.

Discovery Signals for different directions FDM/TDM multiplexed

As previously mentioned, FIG. 7 shows an example of discovery signalsfor different directions, e.g. sectors, multiplexed in a FDM fashionaccording to embodiments herein. The carrier bandwidth is separated intosub-bands and in each sub-band a discovery signal is transmittedbeamformed into an independent direction or into a number of directions.The number of directions is sometimes few, i.e. less than the totalnumber of beamforming patterns possible. Most of the sub-bands willcontain discovery signals pointing into different directions, but anumber of sub-bands may comprise discovery signals pointing into thesame directions. This may be useful if there are directions that aremore “important” than other directions. By the expression “importantdirections” when used herein is meant directions that are more likely tohave receivers 107, e.g. user equipment 106, that may be connected to.The likelihood may be determined by prior knowledge gathered duringprevious connections. Note that not the complete carrier bandwidth needsto be devoted to discovery signals. A part of the carrier bandwidth maybe used which part may be divided into sub-bands carrying discoverysignals.

FIG. 8 schematically illustrates discovery signals for L differentdirections that are multiplexed in frequency according to embodimentsherein. The directions are denoted Direction 1, Direction 2, . . . , andDirection L in FIG. 8. The sub-bands SB1, SB2, . . . , SBL carryingdiscovery signals may be consecutive, as in FIG. 8, but they don't haveto be. The discovery signals may span over a fraction of the totalcarrier bandwidth, as shown in FIG. 8. However, it should be understoodthat the discovery signals may span over the complete carrier bandwidth.

In a simple case, the same discovery signal may be beamformed andtransmitted in each sub-band, i.e. the same discovery signal may betransmitted into the different directions. In some embodiments,information is conveyed together with the discovery signal. Theinformation may comprise e.g. information indicating the beam direction,information indicating the frequency offset to the carrier center,information indicating an access node comprising the transmitter 103.The access node may be a radio network node 102 such as a base station.Further, as previously mentioned, the access node may comprise a radio,i.e. both a transmitter and a receiver.

Transmission of multiple discovery signals, one for each direction, maybe especially beneficial if the radiofrequency design does not permit tocombine Power Amplifier (PA) power of many PAs into one direction. Inother words, the transmission of multiple discovery signals may bebeneficial when only one or few PAs may be combined to radiate into adirection. In one particular design of antenna arrays for a base stationcomprising a radio transmitter 103, multiple PAs are associated withparticular pointing directions by integrating the PA and a one or moreantenna elements into a single radio transmitter, cf. FIGS. 5a and 5b .In general, such an arrangement may impress signals of identicalamplitude into the antenna elements associated with the radiotransmitter, while the phase relationship between the signals beingradiated out of those same elements would be determined by antennaspacing and optionally by phase shifters that precede each element. In adesign with multiple PAs each PA is typically rated at a fraction of theoutput power of the transmitter and only the combined power achievesmaximum output power. If only the power of a few PAs may be combinedinto a particular direction it makes sense to transmit discovery signalsinto other directions simultaneously since the power available for otherdirections may anyway not be reused in another direction.

Some implementations may prefer not to concentrate the full power on afraction of the carrier bandwidth. This design also accommodates suchlimitations.

In embodiments wherein the receiver 107 is a narrowband device, whichlistens to one sub-band it may be located at any direction relative tothe transmitter 103. The transmitter 103 may therefore cycle thetransmitted discovery signals. That is, over time, discovery signalspointing into multiple directions may be transmitted in a sub-band, cf.FIG. 9 for a graphical illustration.

FIG. 9 schematically illustrates discovery signals for differentdirections multiplexed in frequency and repeated over time according toembodiments herein.

A device, such as receiver 107, capable of receiving multiple sub-bandsmay simultaneously listen to multiple sub-bands, e.g. directions, andthus speed up the discovery process. It is apparent that the samearguments regarding the association of PAs with antenna elements in thetransmit direction described above hold for the receive direction in areciprocal fashion, where the receiving aperture of multiple antennaelements may be increased by combining signals from a particulardirection before or after low-noise amplification, cf. FIG. 5c . Theoutputs of each low noise amplifier would typically be down-convertedinto baseband or intermediate frequency signals and digitizedindividually for digital beamforming of multiple groups of associatedantenna elements.

If the energy radiated into one direction is insufficient to enablereliable detection at the receiver 107 the received energy needs to beincreased by accumulating across multiple discovery signals instances.The discovery signals for a particular direction may either be repeatedseveral times in a row or periodically repeated. Both options are shownin FIGS. 10a and 10b , respectively.

FIGS. 10a and 10b schematically illustrate that discovery signals forone particular direction may either be transmitted (twice in FIG. 10a )immediately after each other, and transmitted periodically,respectively, according to embodiments herein. A receiver 107 that maynot detect a single-shot discovery signal may accumulate across multiplerepetitions.

Although FIGS. 9, 10 a, and 10 b only show a natural ordering from 1 toL when cycling through different directions over different sub-bands,there is no limitation whatsoever in using any permutations of suchordering when cycling through different directions. Indeed, having somedifferentiation between the orders may help to create a color code forindividual access nodes, each of which comprises a transmitter 103, solong as the choice of sequences may ensure low probability of collisionbetween signals radiated from several access nodes.

Discovery Signals for Different Directions TDM Multiplexed

According to some embodiments, an alternative design is to use adiscovery signal, e.g. a narrowband discovery signal, which is alwayslocated at the same signaling sub-band, e.g. centered on the totalcarrier bandwidth. Cf. FIG. 11 that schematically illustrates discoverysignals for different directions, e.g. sectors, multiplexed in TDMfashion and transmitted over the same sub-band. In such a design, eachdevice, e.g. each receiver 107, may first find the central sub-band andmay then later on be moved to other sub-bands. However, as illustratedin FIG. 11, the discovery signal does not have to be located in asub-band centered on the total carrier bandwidth.

The discovery signal points into one direction or into few directions.By few directions is herein meant fewer directions than the total numberof beamforming patterns possible. The total number of beamformingpatterns possible is the total number of directions in which thetransmitter 103 may point. To cover all directions the transmitter 103cycles through different directions and transmits discovery signals.

In one simple exemplary implementation, an access node comprising thetransmitter 103, has 2 analog interfaces feeding to independent PAs thatare connected to 2 of 8 possible sectors that are defined by theradiation pattern of 8 horn antennas. The two sectors are addressedusing a switching matrix that couples the PA outputs to particulardirections. This means that two sectors may be simultaneously radiatedfrom at a particular transmission interval. The two sectors correspondto two addressable antenna ports that may be addressed in 4×7=28different ways. Each sector may have a unique discovery signal so thatsectors may be identified at the receiver 107 and so that interferencefrom multiple directions is avoided.

If the output power of multiple PAs may be combined to radiate into thesame direction this design is particularly attractive since it allowsfor such power combining. If implementation allows, the full power of asingle PA may be focused into one sub-band, and thereby increasing theradiated power into the given direction further. However, it should benoted that implementation limitations may suggest not concentrating thefull power of one PA into a single sub-band.

If the energy received with a single discovery signal is insufficientthe receiver 107 may increase the received energy by accumulating acrossmultiple signals. As described above under the section “DiscoverySignals for different directions FDM/TDM multiplexed”, the discoverysignal covering the same direction may be repeated a few times in a rowor periodically transmitted. Such repetition may allow coherentcombining of discovery signals as well as it may aid in synchronization.Combining discovery signals across wide separation of time would proceednon-coherently and may have to assume that fine time synchronization hasbeen achieved through other means.

The discovery signals transmitted into the different directions mayeither be identical or different. In the latter case some informationmay be conveyed with the discovery signal. Some non-limiting examples ofsuch information are an indication of the direction the discovery signalis transmitted into, information indicating the transmitter 103 orindicating an access node comprising the transmitter 103 may becomprised, and information indicating the cycling sequence may also becomprised, which may indicate the next direction to be transmitted in oradditionally defining the transmitter 103 or the access node comprisingthe transmitter 103 as well.

Discovery Signals for Different Directions TDM Multiplexed, FrequencyHopping

In some embodiments similar to some embodiments described above underthe section “Discovery Signals for different directions TDMmultiplexed”, in each time instance one direction or only a fewdirections are illuminated by the discovery signal at a signalingsub-band. Further, TDM is used to multiplex the remaining directions.The only difference between embodiments described in this section andembodiments described above under the section “Discovery Signals fordifferent directions TDM multiplexed”, is that in the embodimentsdescribed in this section frequency hopping is employed for thesignaling sub-band, cf. FIGS. 12a and 12 b.

FIGS. 12a and 12b schematically illustrate discovery signals fordifferent directions, i.e. sectors, multiplexed in TDM fashion andtransmitted over different sub-bands. Further, frequency hopping isapplied, i.e. the frequency sub-band used for the discovery signalchanges over time. The mapping of directions to sub-band changes overtime since some receivers 107 may not be able to receive the totalcarrier bandwidth. To make sure also those receivers 107 may receive thediscovery signal all directions are transmitted from each sub-band overtime. In FIGS. 12a and 12b two frequency hopping periods are shown withchanged sub-band to direction mapping. In this example, the frequencyhopping pattern is the same across periods. However, it should beunderstood that the frequency hopping pattern may be changed.

Frequency hopping implies that the frequency position of the discoverysignal changes over time. In other words, the discovery signal is sweptacross different sub-bands, one at a time. Similar to embodimentsdescribed above under the section “Discovery Signals for differentdirections TDM multiplexed”, this design allows power to be focused intoa given direction thus increasing the coverage area of the discoverysignal. A drawback is that the latency for discovery may be larger dueto the lack of knowledge of the signaling sub-band at the narrowbanddevices.

As previously mentioned, in some embodiments, the location of thesignaling sub-band may not be known to the narrowband device, e.g. thereceiver 107, beforehand, and since the radio front-ends of somenarrowband devices, e.g. receivers 107, may only be able to receive froma limited number of sub-bands, therefore changing, e.g. periodicallychanging, the frequency location of the signaling sub-band allows eachnarrowband device, e.g. receiver 107, to detect the discovery signal bylistening to a fixed sub-band all the time.

Cycling Through Different Directions

In some embodiments described above, cycling through differentdirections has been described. It should be understood that cyclingthrough the different directions and transmitting discovery signals mayeither be done equally frequently into all directions or more often intosome preferred direction. The latter is advantages if one knows thatdevices, e.g. receivers 107, are more often located along somedirections or if one knows that devices, e.g. receivers 107, allocatedin a given sector are more sensitive to latency requirements.

Sequence Design

The receiver 107 may not be aware of which discovery signal, i.e. whichdirection, it is currently receiving and needs therefore to correlatethe received discovery signal with all possible discovery signals. It istherefore important that different discovery signals may be processedefficiently and do not require duplicating processing instances.

In a simple case, the same discovery signal is transmitted in alldirections. However, if some information should be conveyed alongsidewith the discovery signal the same signal may not be transmitted.

One option is to transmit a unitary sequence, e.g. sequence havingconstant magnitude of each sequence sample, and map the sequence samplesto the subcarriers of the discovery signal. Due to its constantmagnitude, such sequence has a low peak-to-average power ratio and thusfacilitates energy-efficient radio transmissions. For the differentdirections, the sequence is mapped with a different cyclic shift to thesubcarriers. If the original sequence is denoted by X_(k), the sequencemapped to the subcarriers of discovery signal for direction d is thenX_((k-Δ) _(d) _()mod N), and Δ_(d) are the sequence length and thecyclic shift applied for direction d, respectively.

One particular choice of unitary sequences is Zadoff-Chu sequences givenby z_(u)(n)=exp(−j^(π)/N^(un)(n+1)) and z_(u)(n)=exp(−j^(π)/N^(un) ² )for odd and even sequence length N, respectively. j is the imaginaryunit sqrt(−1), u is the root sequence index, and 0≤n<N−1.

Another choice of unitary sequences is the Frank sequence, given byz _(v)(n)=exp(−j ^(2π) /M ^(vkl))forn=kM+l, N=M ², 0≤n<N,

M is an integer, j is the imaginary unit sqrt(−1), k and l are integernumbers, and v is the root sequence index.

The Frank sequences may be combined with the Zadoff-Chu sequence toenlarge the available set of different discovery signals. BothZadoff-Chu and Frank sequences have perfect periodic auto-correlationand thus minimize the uncertainty in time synchronization.

Although the description above contains many specifics, they should notbe construed as limiting but as merely providing illustrations of somepresently preferred embodiments. The technology fully encompasses otherembodiments which may become apparent to those skilled in the art.Reference to an element in the singular is not intended to mean “one andonly one” unless explicitly so stated, but rather “one or more.” Allstructural and functional equivalents to the elements of theabove-described embodiments that are known to those of ordinary skill inthe art are expressly incorporated herein by reference and are intendedto be encompassed hereby. Moreover, it is not necessary for a device ormethod to address each and every problem sought to be solved by thedescribed technology for it to be encompassed hereby.

When using the word “comprise” or “comprising” it shall be interpretedas non-limiting, in the meaning of consist at least of.

When using the word action/actions it shall be interpreted broadly andnot to imply that the actions have to be carried out in the ordermentioned. Instead, the actions may be carried out in any suitable orderother than the order mentioned. Further, some action/actions may beoptional.

The embodiments herein are not limited to the above described examples.Various alternatives, modifications and equivalents may be used.Therefore, the above examples should not be taken as limiting the scopeof the invention, which is defined by the appending claims.

The invention claimed is:
 1. A method, in a base station, fortransmitting synchronization signals to a user equipment, wherein thebase station and the user equipment are comprised in a radiocommunications system, the method comprising: transmitting two or moresynchronization signals over two or more directions; and wherein eachsynchronization signal is configured to span over a fraction of acarrier bandwidth, the carrier bandwidth is separated into sub-bands,and each of the two or more synchronization signals is configured tospan over one sub-band.
 2. The method of claim 1, wherein transmittingthe two or more synchronization signals comprises transmitting the twoor more synchronization signals in two or more symbols.
 3. The method ofclaim 2, wherein transmitting the two or more synchronization signalsfurther comprises: multiplexing, in the frequency-domain, the two ormore synchronization signals for the two or more directions; andtransmitting the multiplexed two or more synchronization signals in thetwo or more symbols.
 4. The method of claim 1, wherein transmitting thetwo or more synchronization signals further comprises: multiplexing, inthe time-domain, the two or more synchronization signals for the two ormore directions; and transmitting the multiplexed two or moresynchronization signals over the same sub-band.
 5. The method of claim1, wherein transmitting the two or more synchronization signalscomprises transmitting the two or more synchronization signals togetherwith information relating to at least one of: information indicating abeam direction; information indicating a frequency offset to a carriercenter; and information indicating the base station.
 6. The method ofclaim 1, wherein transmitting the two or more synchronization signalscomprises cyclically, over time, transmitting the two or moresynchronization signals into the two or more different directions.
 7. Abase station for use in a radio communications system, the base stationconfigured to transmit synchronization signals to a user equipment, thebase station comprising: one or more processing circuits operative tocontrol the base station to transmit two or more synchronization signalsover two or more directions; and wherein each of the two or moresynchronization signals is configured to span over a fraction of acarrier bandwidth, the carrier bandwidth is separated into sub-bands,and each of the two or more synchronization signals is configured tospan over one sub-band.
 8. The base station of claim 7, furtheroperative to: transmit the two or more synchronization signals in two ormore symbols.
 9. The base station of claim 8, further operative to:multiplex, in the frequency-domain, the two or more synchronizationsignals for the two or more directions; and transmit the multiplexed twoor more synchronization signals in the two or more symbols.
 10. The basestation of claim 7, further operative to: multiplex, in the time-domain,the two or more synchronization signals for the two or more directions;and transmit the multiplexed two or more synchronization signals overthe same sub-band.
 11. The base station of claim 7, further operativeto: transmit the two or more synchronization signals together withinformation relating to at least one of: information indicating a beamdirection; information indicating a frequency offset to a carriercenter; and information indicating the base station.
 12. The basestation of claim 7, further operative to: transmit, cyclically, overtime, the two or more synchronization signals into different directions.13. A method, in a user equipment, for receiving synchronization signalsfrom a base station, wherein the base station and the user equipment arecomprised in a radio communications system, the method comprising:receiving, from the base station, at least a first synchronizationsignal of two or more synchronization signals that have been transmittedinto two or more directions; and wherein each of the two or moresynchronization signals is configured to span over a fraction of acarrier bandwidth, the carrier bandwidth is separated into sub-bands andeach of the two or more synchronization signals is configured to spanover one sub-band.
 14. The method of claim 13, wherein receivingcomprises receiving two or more synchronization signals in two or moresymbols.
 15. The method of claim 14, wherein the two or moresynchronization signals have been transmitted by: multiplexing, in thefrequency-domain, the two or more synchronization signals for the two ormore directions; and transmitting the multiplexed two or moresynchronization signals in the two or more symbols.
 16. The method ofclaim 13, wherein the two or more synchronization signals have beentransmitted by: multiplexing, in the time-domain, the two or moresynchronization signals for the two or more directions; and transmittingthe multiplexed two or more synchronization signals over the samesub-band.
 17. The method of claim 13, wherein receiving comprisesreceiving at least the first synchronization signal together withinformation relating to at least one of: information indicating a beamdirection; information indicating a frequency offset to a carriercenter; and information indicating the base station.
 18. The method ofclaim 13, wherein the receiving comprises cyclically, over time,receiving the first synchronization signal, which has been transmittedin different directions.
 19. A user equipment for use in a radiocommunications system, the user equipment configured to receivesynchronization signals from a base station, the user equipmentcomprising: a receiver configured to receive, from the base station, atleast a first synchronization signal of two or more synchronizationsignals that have been transmitted into two or more directions; andwherein each of the two or more synchronization signals is configured tospan over a fraction of a carrier bandwidth, the carrier bandwidth isseparated into sub-bands, and each of the two or more synchronizationsignals is configured to span over one sub-band.
 20. The user equipmentof claim 19, further configured to receive the first synchronizationsignal in two or more symbols.
 21. The user equipment of claim 20,wherein the two or more synchronization signals have been transmittedby: multiplexing, in the frequency-domain, the two or moresynchronization signals for the two or more directions; and transmittingthe multiplexed two or more synchronization signals in the two or moresymbols.
 22. The user equipment of claim 19, wherein the two or moresynchronization signals have been transmitted by: multiplexing, in thetime-domain, the two or more synchronization signals for the two or moredirections; and transmitting the multiplexed two or more synchronizationsignals over the same sub-band.
 23. The user equipment of claim 19,wherein the receiver is further configured to: receive at least thefirst synchronization signal together with information relating to atleast one of: information indicating a beam direction; informationindicating a frequency offset to a carrier center; and informationindicating the base station.
 24. The user equipment of claim 19, whereinthe receiver is further configured to: receive cyclically, over time,the first synchronization signal, which has been transmitted indifferent directions.